This invention relates generally to a system and method for providing a motion-compensated drilling rig platform. More particularly, the invention relates to an automated system and method which can be used to control marine riser disconnection events and riser tensioner wireline breaks in conjunction with such a platform.
Drilling operations conducted from a floating vessel require a flexible tensioning system which operates to secure the riser conductor between the ocean floor (at the well head) and the rig, or vessel. The tensioning system acts to reduce the effects of vessel heave with respect to the riser, control the effects of both planned and unplanned riser disconnect operations, and to mitigate the problems created by unexpected breaks or faults in the riser (hereinafter a xe2x80x9cdisconnect eventxe2x80x9d).
Riser tensioner devices, which form the heart of the tensioning system, have been designed to assist in the management of riser conductors attached to drilling rigs, especially with respect to movement caused by periodic vessel heave. A series of these tensioners, connected to the riser using cables and sheaves, react to relative movement between the ocean floor and the vessel by adjusting the cable length to maintain a relatively constant tension on the riser. Any number of tensioners, typically deployed in pairs, may be used to suspend a single riser from the vessel.
Unexpected events may occur during offshore drilling operations. These may be realized in the form of tensioner wireline breaks, severe storms, or other circumstances which require the vessel/rig operator to act quickly to adjust the tension applied to the riser. The riser may also become disconnected from the wellhead for various reasons.
The need to respond to an unexpected riser disconnect event, or tensioner wireline break, and manage the recoil tension or xe2x80x9cslingshotxe2x80x9d effect on the vessel induced thereby, provides the motivation to develop an automated system and method to control the movement of individual tensioners. The system and method should operate by managing the tension applied to the riser using the cables attached to the riser and the riser tensioners in response to sensing an irregular travel velocity experienced by one or more of the tensioners, such as may be caused by a disconnect event or tensioner wireline break. Thus, the system and method should be simple, robust, and fully automatic, such that system elements are capable of responding to and continuously managing a disconnect event or tensioner wireline break in an automated fashion more rapidly and reliably than is possible using human operators.
In one embodiment, the automated riser recoil control system includes a plurality of riser tensioners, a vessel heave measurement system, and a control processor in electrical communication with the heave measurement system and the riser tensioners. Each tensioner includes a piston travel indicator which provides a piston travel signal to the processor, while the vessel heave measurement system provides a heave velocity signal to the processor.
The processor monitors each of the piston travel signals along with the heave velocity signal so as to be able to determine whether a preselected number of piston travel velocities (determined from the piston travel signals) exceed the vessel heave velocity by some critical velocity difference. For example, if sixteen riser tensioners are used to suspend the marine riser from the heaving vessel, and at least four of the tensioners show a piston travel velocity which exceeds the heave velocity by more than about one foot per second (value is typically between about 4-6 feet/second cable speed or about 1.25 feet/second tensioner piston velocity), then the processor, which is in controlling communication with each one of the riser tensioners, can react by controlling the force applied to the riser by controlling the rate of fluid flow within one or more of the tensioners.
Typically, each of the riser tensioners includes an accumulator chamber (blind end of the tensioner) and a piston bore chamber (rod end side of the tensioner), and the fluid flow is controlled within the piston bore chamber. To control the fluid flow, an orifice-controlled fluid valve is typically placed in fluid communication with the piston bore chamber. To further control movement of the tensioner, an air shutoff valve is typically placed in fluid communication with the accumulator chamber and a bank of high pressure air cylinders. Timers may be applied to adjust the time within which the orifice-controlled fluid valves and air shutoff valves are closed. Finally, to prevent extreme movement of the tensioner, a fluid volume speed control valve may also act to limit the volumetric rate of fluid flow in the piston bore chamber upon sensing an extreme fluid flow rate within the tensioner.
In another embodiment, a method for adjusting at least one of the tension forces applied by the tensioners to the riser includes the steps of determining the piston travel velocity for each riser tensioner, measuring the heave velocity of the vessel, calculating the velocity differences between each of the piston travel velocities and the heave velocity, and adjusting the tension force after determining that some preselected number of the velocity differences exceeds a preselected critical velocity difference (selected by the operator). Again, control of the tension force is typically effected by throttling the rate of at least one fluid flow within one or more of the plurality of riser tensioners. Air shutoff valves, orifice-controlled fluid valves, and fluid volume speed control valves are all used as previously described.